Mass spectrometer



0d 23, 1956 W. H. WELLS E1' AL 2,768,304

MAss sPEcTRoMETER 2 Sheets-Sheet 2 Filed Oct. 15, 1951 V1.5 wfu. MMM mw.1 /.MM Mm MU. /b/ MW United States yPatent O masses MAssrsrrzecrrwrvnsma `William H. Wells and William C. Wiley, Detroit, Mich.,

assignors to Bendix Aviation Corporation, Detroit, Mich., a corporationof Delaware This invention relates to mass spectrometersand moreparticularly to mass spectrometers for, and methods of, distinguishingbetween the masses of dilerent ions `b 'y measuring their times offlight. The invention ,is especially adapted to provide a relativelysharp diiere'ntiation between zions of different masses.

vIn some mass spectrometers now in use, pulses of ions are formed fromthe different molecules of gas in an unknown mixture and are subjectedto va predetermined Iforce so as tobe accelerated through apredetermineddistance. The ,ions of relatively light mass are Vgiven agreateracceleration by the predetermined vforce than the ions .-of heavymass and are collected atthe end ofthe ,predetermined distance beforethe Aions of heavy mass. Thus, by measuring the times required fordiierent ions `in t-he pulse 'to travel through the predetermineddistance, the massesof the ions can be determined.

Difficulties in measurements arise in thespectrometers now in useandprevent the measurements fromkbeingdas l z;1c c t 1rateandmeaningfulas might bedesired. For example, isinceuthe nite Widthofthe pulse may pause Ividual ions of a given massto be located at diterent positions in the pulse before the,ionsareaecelerated wards thecollectorplate, all ions of the givenA mass arenot c oiiected at thesame time. vIn addition, thethermaLand other energy of theions imparts arandom motionto the ions before the application of any eXternalforce 1sothat somedons of -a given mass are moving towards the Ycollector plateat the `time that the predetermined f orceis .imposed and otherions ofthe same mass aremoving away from the collector plate4 at that instant.Thus, thermaland other Ienergy ofthe ,ions als-O prevents all ions ofagiven in ass from, being, simultaneously rcoilected in spectrometersnow in use. v l I This invention provides la mass spectrometer in whicherrors resulting from the different positioning and random motion ofindividual ions are minimized. YThe errors are minimized by the.utilization .of an accelerating gijidhaving an extended width andby thedispositionof the collector plate at va predetermined vdistancef-ronlfthe. starting apoint of the ions.

vAn object of this invention is to provide a mass spectrometerffordistinguishing between ions of different mass by measuring the timerequired for the ions :to travel through a predetermined distance.

Another object is to provide a mass spectrometer yof ,the abovecharacter forproviding a relativelyv sharp dif- Iierentiation betweenions of diierent mass.

.A -furtherobjectis to provide amass spectrometer of the above characterutilizing 4an acceleratinggrid having an ,extendedwidth so as to improvethe,res ponse char ac teri stics .ofy the spectrometer. Y

@Stillanother object is to, provide amass; spectrometer of the abovecharacter having a collector plate disposed Vv.at a.predetermineddistance from the ,grid toimprove the response characteristics of the,spectrometer.

`Astill Vfurthenobject isto .providemethodsof thefabove 2,768,304Patented Oct. 23, 1956 rice character for providing a relatively sharpdifferentiation betweenions of different mass.

Qther objects and advantages will be apparent from a detailedVdescription of the invention and from the appended drawings and claims.

In the drawings:

Figure l is a view illustrating somewhat schematically the mechanicaland electricalv features constituting one embodiment of the invention,vthe mechanical features Ybeing'shown in perspective and some of theelectrical features being shown in block form;

vFigure 2 is an enlarged sectional view of the gridstructure'constituting a part of the embodiment shown in Figure 1, thesec-tion being taken substantially on the axis of the structure; and iFigure 3 4is a schematic perspective view illustrating somewhatschematicallyanother embodiment of the inventon.

In `the embodiment of the invention shown in Figure 1, a cathode`10 madefrom a wedge-shaped strip of a suitable material such as tungsten isprovided. A control grid y1 2 is disposed relatively close to thecathode 1t) and isprovided with a vertical slot 14 whose median positionlies in substantially the s ame horizontal plane A'as the cathode. Anaccelerating vgrid 1 6 is positioned relatiyelyclose to the control grid12 on vone side and to a shield grid 18 .on the other Vside and issubstantially in alignment with both grids. Vertical slots l2 0 an d2 2corresponding substantially in shapeva'nd position to the Slot 14impro-vided in the grids .16 ,and 18, respectively- A verticallydisposed backing plate 24 is provided to the rear of .the slots 1 4, 20and.2 2 and s ubstantiallyrin parallel `with the dow of electrons fromythe cathode 10 through the slots. A non-conductive container 2 6 hav-1ng .-a relatively great axial length and a relatively small widthextends in a horizontal plane lfrom the plate .24.

The container L26 preferably has'a circular crosssection in a directionperpendicular to its axisbut may have other sectional contigurations,suchasa squarecross-sec- Ytion.

' vBores 28 4areprovided inthe container 2 6 substantially in alignmentwith the median positions of the Vslots 14, 2.0V and The bores 28 serveto provide a passageway for the flow of electrons .transversely throughthe container 26 vto a collector plateA 29 positioned on'the far sideofthe container substantially in parallel with the ,grids 12,16 and 1 8.A grounded conductive sleeve 3 0 "having holes corresponding in positionto the bores 2 8 ts on the container 26. T he sleeve l30 eliminates anyeiects trom ,electrical charges which result when electrons fromthe'electron stream Virnpinge on theeontainer instead of passing throughthe bores. vI-lolesl are also madein the Vbot-tom ofthe container zoandthe sleeve @substantially in vthesarne plane as the vbores 2,8tofprovide forthe introduction of gasthrough a conduit 32. The conduitin turn extends from a receptacle 34 adapted to yhold molecules of theditierent gases constituting an unknownmixture. I

'A ,plurality 'of conductive grids 3 6 made from a suitably meshedmaterial extend at spaced yintervals across the b ore in the container26 substantially in perpendicular relationship fto the'axis of rthecontainer. The grids sive grids. The grid furthest removed from theplate 24 may be grounded.

A collector plate 42 is positioned substantially parallel to the plate24 and grids 36 at a distance from the end of the container 26approximately twice as great as the length of the container itself. Atime indicator 44 such as an oscilloscope, is connected to the collectorplate 42 to indicate the relative times at which the ions of differentmass arrive at the collector plate. Since the collector 42 actuallyserves as an ion detector, other types of detectors may be used.

In the steady-state condition, the control grid 12 and the acceleratinggrid 16 have positive voltages applied to them from suitable outputterminals of a direct power supply 46, and the collector plates 29 and42 have slightly positive voltages applied to them from other terminalsof the power supply. The cathode 10, the shield grid 18, the backingplate 24 and the grids 36 are substantially at ground in thesteady-state condition.

Because of the positive voltage on the grid 12 relative to the voltageon the cathode, electrons emitted by the cathode are accelerated towardsthe grid in the steadystate condition. The electrons are not acceleratedbeyond the grid 12, since the grid 16 has approximately the samepositive voltage as the grid 12, and the grid 1S is substantially atground. As a result, any electrons which pass through the bores 28 inthe container 26 do not have sulicient energy to ionize molecules of gasintroduced into the container through the conduit 32.

At predetermined times, negative voltage pulses of substantially equalmagnitude are applied to the cathode and the control grid 12 from thepulse forming circuit 40. Since the grid 12 is still more positive thanthe cathode 10, the electrons emitted by the cathode continue to beattracted towards the grid. The electrons are further considerablyaccelerated in the region between the grids 12 and 16 because of thepositive voltage which exists on the grid 16 relative to the voltage onthe grid 12 during the pulses. The additional acceleration imparted tothe electrons by the grid 16 causes the electrons to travel through thebore 28 into the container 26 with suicient energy to ionize some of thegas molecules introduced into the container.

Because of their positive charge, the ions produced by the collisionbetween electrons and gas molecules are retained within the negativefield created by the stream of electrons owing towards the collectorplate 29. By retaining the ions within the electron stream, a pulse ofions having a relatively narrow width is obtained. The electron streamis relatively narrow because of the collimating action provided by theslots 14, 20 and 22 and the collimating action which may be provided bya suitable magnetic eld (not shown).

In addition to producing an ion pulse having a relatively narrow width,the relatively large charge of the negative eld causes the number ofions forming the pulse to be considerably increased over the number ofions comprising a pulse in spectrometers now in use. The creation of anion pulse having a relatively narrow width and a relatively large numberof ions produces an increase in the sensitivity with which the massspectrometer responds to ions of different mass. The formation of theelectron stream and the retention of ions within the stream aredisclosed in detail in co-pending application Serial No. 221,554, filedApril 18, 1951, by Ian H. Mc Laren and William C. Wiley, now Patent No.2,732,500.

The ions retained within the negative field are produced during theapplication of the negative pulses on the cathode 10 and grid 12. Atapproximately the same time as the negative pulses on the cathode 10 andgrid 12 are discontinued or at a slightly later time, positive pulsesare applied to the backing plate 24 and' the grids 36. Equipment forproducing a pair 4of pulses separated from each other by a relativelyshort time is. well known to persons skilled in the art and may beeither purchased or built. For example, the Berkeley Scientific Companyof Richmond, California, manufactures equipment for producing a pair ofpulses which may be separated from each other by a variable period oftime dependent upon the adjustment of a knob. This equipment isdesignated by Berkeley as Model 902 of its double pulse generator. If itis not desired to purchase equipment which provides a pair of pulsesseparated from each other by a variable period of time, such equipmentmay be built in accordance with principles outlined on pages 223 to 238,inclusive, of volume 20 entitled Electronic Time Measurements of theRadiation Laboratory Series prepared by the Massachusetts Institute ofTechnology.

The voltage pulses on the plate 24 and the grid 36 cause the ions to beaccelerated through the container 26 towards the collector 42. Since theions of relatively light mass are accelerated more by the predeterminedforces applied by the grids 36 than the ions of heavy mass, they reachthe collector plate 42 before the ions of heavy mass. By measuringV thetimes at whichthe diierent ions reach the collector plate, the masses ofthe ions can be determined.

We have found that, in spite of the narrow Width of the ion pulse,errors in measurement result in spectrometers now in use from thedifferent disposition of individual ions in the ionization region. Forexample, some ions of a given mass may be located spatially at the backof the pulse and other ions of the same mass may be positioned at thefront of the pulse. This causes individual ions of a given mass toarrive at the collector plate before other ions of the same mass andclouds the measurements which are obtained.

Errors in measurement also result in spectrometers now in use from therandom motion imparted to the ions by the thermal and other energy ofthe ions before the application of any external forces. For example,some ions of a given mass may be moving towards the collector plate atthe instant that a voltage pulse is applied to the ion acceleratinggrid, and other ions may be moving away from the collector plate at thatinstant.

We have also found that the errors resulting from the differentpositioning and the random motion of individual ions can be minimized ifthe collector plate is placed at a predetermined distance from theaccelerating grid. This distance has been found by analysis to beapproximately twice the distance between the grid and the ion pulsebefore the pulse is accelerated towards the grid when the ion pulse hasa relatively narrow dimension in the direction of the grids 36. Forincreased lengths of the ion pulse, the position of focus is given asQ=the position of optimum ion focus;

S1=the distance from the last grid 36 to the end of the ion pulsenearest the backing plate 24;

S2=the distance from the last grid 36 to the end of the ion pulse mostremoved from the backing plate 24.

The term focussing as used herein refers to the action of bringing theions of each mass together into relatively tight groups in the directionof their movement. As such, the term focussing refers to the action ofbunching the ions of each mass in the direction of their movement sothat they are collected in a minimum amount of time. Since the distancebetween the pulse and the grid is relatively small, the distance betweenthe grid and collector plate is also small. This short distance prevents ions of diterent mass from becoming sufficiently resolved in spacebefore they reach the collector plate and causes the spectrometer togive somewhat inaccurate results.

This invention focusses the ions of each mass in the direction of theirtravel by disposing a detector such as the collector 42 at a positionapproximately twice as gees-sda fas-awayf-fidm die-fleet grld 's6-asLdie distance' "'etween /t'h .electroni streamL an tlielast-VI gridi*Elievr invention y produce inatt-,rial separation Between the ions of?dittfen' massi' byf 'eiec'tively'increasing'the length of die.aeeelratinglregien. This isy accomplished by prengafplurality of*equally spaced?! grids sfi-a'nd-by appl ng voltagepulses` of;progressively' decreasing amplitude-between successivelv paies off'grids. The-ions nien move through the grid with a substantiallycons-tarifa@- celeration. I Because of the substantially constantaccelerationriitipartedttotheions; tlie' grids in* eliect provide anelectricalg eld havingvasubstantially constant intensity over anextended width. This in turn Vcauses the .dist/ance between the endv ofthe lastgrid 36* and: the collector plate 4'2 to be considerablyincreased in order tra-obtain. adistance which Ais approximately twicethe distance between the ion pulse in its stationary position and thelast gridl'3'6.

BeeauSe-of the extendedlengfh on the electr-ical i'eld and3 thedistant-positioning? ofthe? collectorplate 42,- the ions require atleast as long:- a titne-toreach." the collector plate as in thespectrometers now in use. By placing the collector plate at the optimumdistance from the last grid 36 without reducing the time of ion travel,ions of a given mass are focussed in space better than in spectrometersnow in use. This in turn causes the sharpness and accuracy of themeasurements obtained to be considerably increased.

Since the ultimate object is to obtain an electrical iield which issubstantially constant over an extended length, the operation of thegrid structure shown in Figure l can probably be enhanced by increasingthe number of grids and correspondingly decreasing the distances andvoltages between them. The grid structure may also be used so that ringssimilar to the ring 38 are used without any wire mesh.

It should be realized kthat other structures than that disclosed abovemay be used to form a relatively narrow ion pulse for passage throughthe container 26. It should be realized also that the ions may beaccelerated through the container by applying pulses of negative voltageto the different grids. In this way, the ions are attracted towardssuccessive grids rather than being repelled from one grid to the next.

A modification of the invention disclosed above is illustrated in Figure3. In this embodiment, a container 70 formed from a thin sheath ofconductive material is used. The end of the container near the collectorplate is grounded and the end of the container near the backing plate isadapted to receive a pulse of positive voltage from a pulse formingcircuit 52. Current ows axially along the sheath and produces a voltagewhich gradually decreases along the axial length of the sheath.

It should be appreciated that the plurality of grids 36 are provided toproduce an electrical iield of extended length. A iield of extendedlength is produced so that the collector 42 can be positioned arelatively great distance from the backing plate 24.

Although this invention has been disclosed and illustrated withreference to particular applications, the principles involved aresusceptible of numerous `other applications which will be apparent topersons skilled in the art. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims.

What is claimed is:

l. A mass spectrometer, including, means for obtaining a plurality ofions, an ion detector, a grid assembly positioned between theion-obtaining means and the ion detector to provide a substantiallyconstant acceleration of the ions through a first region equal indistance to substantially one-half the distance between the end of thefirst region and the detector, and means for indicating the relativetimes at which the ions of different mass are detected.

2. A mass spectrometer, including, means for obtaining plnraiityef ions;an .ien detecter', a" grid assenibiy positioned between theion-obtaining means and tle'cl`e teeferj and having extended lengrlrniii the direction of thede'tctr equal' to' substan'tiallr 'one-half thedistance between-'the'I endJT of die grid' assemblyv and; die:Ivdetector, means f or producing an electric 'eld through the Yex"- tendedlen'g'tli ofthe" grid' assembly' to provide' a constant 'a'c'celerationon the io'ns', pand' rne'a'ns for providing v an indication of' thelrelative times at whieli'dieions oridiirent mass are detected.

3. A- mass spectrometer, including, means for obtaininga` pluralityw ofions; anion detector, a grid assembly positioned between theion-obtaining means and"y 'die derector andi equal inv distancesubstantially to one `lia-1l? die distance between the end yof" thevvgrid'as's'ernblyancf tli'e detector, means for` applying voltage*pulses' of predeterrdin'e'dmagnitudeen nie-'grid' assemblyfo lpr'odee asub- -stantially constant acceleration of tlre ions througlr die gridassembly, arianne-ans for indicating-fthe rela-rive finies at. whicht-le ions-of: different massare deter-midi.h

4. A- m'ass spectrometer, including, meansfon obtain'- ing a pluralityof ions, means for providing an electric iield of substantially constantmagnitude through a first region to provide a substantially constantacceleration of the ions through the region, an ion detector positionedat a distance past the end of the first region corresponding tosubstantially twice the distance of travel of the ions through theregion, and an indicator for showing the relative times at which theions of different mass are detected.

5. A mass spectrometer, including, means for obtaining a plurality ofions, means for providing a substantially linear and constantacceleration of the ions through a rst region, an ion detectorpositioned at a distance past rthe end of the iirst region correspondingto approximately twice the distance of ion acceleration through thefirst region, and means for indicating the relative times at which theions of diiierent mass are detected.

6. A mass spectrometer, including, means for providing p-ulses of ions,an ion detector, a grid structure positioned between the ion-providingmeans and the detector, means for applying voltages to the gridstructure to produce a movement of the ions in each pulse towards thedetector, the detector being positioned at a distance past the end ofthe grid structure corresponding to approximately twice the distance ofacceleration of the ions by the grid structure, and means for indicatingthe relative times at which the ions of different mass in each pulse aredetected.

7. A mass spectrometer, including, means for obtaining a pulse of ionsin a coniined region, means for providing an electric field ofsubstantially constant magnitude through a first region to provide asubstantially constant acceleration of the ions through the region, anion detector positioned at a distance past the efnd of the iirst regioncorresponding to substantially twice the square root yof the distancefrom the end of the rst region to the remote end of the confined regionmultiplied by the distance from the end of the first region to the nearend of the conlined region, and an indicator for determining therelative times at which the ions of dilierent mass are detected.

8. A mass spectrometer, including, means for providing pulses of ions offinite width, an ion detector, a grid structure positioned between theion-providing means and the detector, means for applying voltages to thegrid structure to produce a movement of the ions in each pulse towardsthe detector, the detector being positioned at a distance past the endof the grid structure corresponding to substantially twice the squareroot of the distance between the end of the grid structure and theremote end |of the ion pulse multiplied by the `distance between the endof the grid structure and the near end of the ion pulse, and anindicator for determining Ithe relative times at'which the ions ofdilerent mass in each pulse are detected. A

9. A mass spectrometer, including, means for providing a plurality ofions, an ion detector, a plurality of grids disposed at spaced intervalsbetween the ion-providing means and the detector, means for applyingprogressively more negative voltage pulses to the successive grids inthe plurality to produce a substantially constant electric fieldthroughout lthe extended region defined by the grids for movement of theions past the grids and towards the detector, the detector beingpositioned at a focal distance past the end of a last grid in theplurality to detect the ions of each mass at substantially the sameinstant of time, and means for indicating Ithe relative times at whichions of dierent mass are detected.

10. A mass spectrometer, including, means for providing a plurality ofions, an ion detector, a plurality of grids disposed at spaced intervalsbetween the ion-providing means and the detector to provide an electriciield of substantially uniform intensity between the ion-providing meansand a last grid in the plurality upon the application of particularvoltage pulses to the grids in the plurality, means for applying theparticular voltage pulses to the grids in the plurality to produce amovement of the ions past'the grids and towards the detector, thedetector being positioned at a distance past the last grid equal tosubstantially twice the distance between the ionproviding means and thelast grid, and means for indicating the relative times at which ions ofdifferent mass are detected.

References Cited in the file of this patent UNITED STATES PATENTS2,476,005 Thomas July l2, 1949 2,535,032 Bennett Dec. 26, 1950 2,582,216Koppius Jan. 15, 1952 2,685,035 Wiley -Tilly 27, 1954 OTHER REFERENCESAn Ion Velocitron, by Cameron et al., published in The Review ofScientilic Instruments, vol. 19, No, 9 of September 1948, pages 605-607.

